Depending on the time you have there are different times you may put your family together.
You might make your complete family early in December so it is complete and ready to start with you on the 16th December.
You may create your family as you move through the activities each night. You could make the character or characters associated with the reflection for the day.
Schools and groups, you could ask individuals or small groups to make the family.
Whichever way you do it, it is helpful to have everything you need before you start so you aren't hunting for materials every day of the Posada.
How do we make the Holy Family?
The short answer is however you want. You can be as creative or uncreative as you wish, the idea is that you can enjoy it together.
Church - If you are running this for families or for a school. You may want to provide the Holy Family, the resources to make it or the ideas for them to be creative. Especially in schools it will be important to have a conversation about what is possible.
School - If you are running this yourselves, could you make this an art project or homework challenge? Are there parents who would like to help (and then isolate them before they go into classrooms)
Family - Be creative but be sensible. If you are doing a school run or full days at work, will you have time to make a character and do the activity each day? If yes, go for it. If not, work on the family beforehand. It could be a weekend activity, a family competition or a way of involving wider family you cannot see.